Space is simply the way in which our minds see the world; that is, it is purely mental and not really outside us. The corollary to this is that as all things have their being in space, they must likewise have their being in the mind. But mind alone can only entertain mental visitors; it is too subtle to receive non-mental materials. Mind cannot receive that which is wholly dissimilar to it. Therefore all things must enter it as ideas only.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 160